I would say the input Opamps or some bad solder joints.
Never know till it's put in a test setup.

Quote from: g1 on May 13, 2014, 03:56:55 PM
  Put a patch cord between preamp out and power amp in (dry), any difference?

That had already been checked. I'm an experience amp tech, 

Well, I found the culprit.......
Schematic calls for LF356 in the inputs of the normal and reverb channels, there were LF351's in there. I don't know if that was the cause or if the chip had become faulty. But the LF351 in the normal ch. caused it. I removed it and the oscillation disappeared. Went ahead and ordered the correct opamps for both inputs.

Another side issue, after I corrected the oscillation, was severe distortion on the output. So I rebiased the idle current and offset Voltage to factory specs.
Plugged it back in and found a rubbing speaker, too!

This guy must have let this thing sit in storage for long time or ran it into the dirt!

It'll be sweet sounding after I get done with it! 8|

A Customer brought me a Lab Series L5 with a high frequency oscillation around 5k Hz.
The noise is on the negative and positive rail voltages going to the Opamps, but I've isolated the power supply and power amp and they don't have the problem.

The noise is coming from the preamp and increases with both channel volumes and master volume. Weird side effect, the frequency slightly changes with the channel volumes. Ie, ..Normal channel frequency increases, Reverb Ch, frequency decreases when increasing volume. I've started pulling opamps and still haven't found the issue.

Any ideas?